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Full text of "The Cambridge Natural History"

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grow to a very great length, extending back within the jaw to
near the hinder part of the skull. These teeth are reinforced in
the upper jaw by a small second pair in the £ğagomorpha only.
The incisors are chisel-shaped, and often brown or yellow upon
the outer face, as is the case also with some Insectivores. This
peculiar shape, and their strength, renders them especially capable
of the gnawing action which characterises the Bodents. It has
been pointed out that where the incisors are wider than thick,
the gnawing powers are feebly developed ; and that on the con-
trary, where these teeth are thicker than wide, the animals are
good gnawers. The incisors have often an anterior groove, or it
may be grooves.

FIG. 232,-

-Molar teeth of Rodents.     A, of Capybara (Hydrochoerus) ; B, of Squirrel
; C, of Ctenodactylus.    (After Tullberg.)

The cheek teeth vary in number from two (Jlydromys) to six
(Tlabbit) on each side of the two jaws. Four is the prevailing
number outside the large division of the Hat-like [Rodents.
They are often set at an angle to the horizontal plane of the jaw,
looking outwards and obliquely to its longitudinal axis; the in-
dividual teeth too are not unfrequently bowed in form, remind-
ing us of those of Toxodon. This of course only occurs in those
genera which have hypselodont teeth. The pattern of the teeth
varies much, and the different forms recall the teeth of more
than one other group of mammals. They are either buiiodont
or lophodoiit. In many cases the tooth is encircled with a ridge
of enamel, which is either almost simple or has a more com-
plicated contour; such teeth are by no means unsuggestive of the
Toxodonts. Some of the lophodont molars are by no means
unlike those of the Proboscidea. Tri tfciurtts vulywris the